Create regular expression match exact word with space followed by string length 8

Question

I am very new to regular expressions , I want to check in a sentence that matches with exact given word followed by space and a alphanumeric string starts with number and length of that string should be 8.From the below expression i am getting 1 as result and matches PO. But I am not getting how to create the rule to match PO followed by space and starts a string with first letter numeric and length with 8 characters.Any help would be greatly appreciated.

$header = 'Amazon.fr PO 2YCLN4EM';
$k = preg_match("/[PO]*/", $header);
echo $k;

Answer 1

Try as follows

Use \\d and Limiting Repetation for match exact 7 times. DEMO

/PO\s\d\w{7}/

Answer 2

$header = 'Amazon.fr PO 2YCLN4EM';

if (preg_match('/PO\ \d\w{7}/', $header, $matches)) {
    print_r($matches);
}

Note that the function only returns whether you have a match or not. The third parameter contains the matches.